Imaging device

ABSTRACT

An imaging device includes: pixels arranged one-dimensionally or two-dimensionally, each of the pixels including an electrode that is electrically connected to the other pixels, a charge capturing unit that is separated from the other pixels, and a photoelectric conversion layer that is located between the electrode and the charge capturing unit, the photoelectric conversion layer being continuous among the pixels. The photoelectric conversion layer contains semiconductor carbon nanotubes, and one of a first substance and a second substance, the first substance having an electron affinity larger than that of the semiconducting carbon nanotubes, the second substance having a ionization energy smaller than that of the semiconductor carbon nanotubes.

This is a Continuation of application Ser. No. 16/831,231, filed Mar.26, 2020, which is a Continuation of application Ser. No. 15/729,739,now U.S. Pat. No. 10,672,836) filed Oct. 11, 2017, which is a BypassContinuation of International Application PCT/JP2016/002917, filed Jun.17, 2016, which claims priority to Japanese Patent Application No.2015-137063, filed on Jul. 8, 2015; Japanese Patent Application No.2015-137064, filed on Jul. 8, 2015; and Japanese Patent Application No.2016-082890, filed on Apr. 18, 2016, the entire disclosures of which arehereby incorporated herein by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to a stacked imaging device.

2. Description of the Related Art

In recent years, stacked image sensors in which photoelectric conversionelements are provided on semiconductor substrates have been realized. Instacked image sensors, photoelectric conversion layers in thephotoelectric conversion elements can be formed of materials that aredifferent from the materials of the semiconductor substrates. Thus, thephotoelectric conversion layers can be formed of inorganic materials ororganic materials that are different from semiconductor materials of therelated art, such as silicon, and image sensors having physicalproperties or functions that are different from those of image sensorsof the related art, such as sensitivity to a wavelength band that isdifferent from the wavelength band of the related art, can be realized.For example, Japanese Unexamined Patent Application Publication No.2003-234460 discloses an image sensor including photoelectric conversionlayers that have sensitivity to two or more different wavelength bands,thereby having high light utilization efficiency.

SUMMARY

One non-limiting and exemplary embodiment provides a novel stackedimaging device.

In one general aspect, the techniques disclosed here feature an imagingdevice including: pixels arranged one-dimensionally ortwo-dimensionally, each of the pixels including an electrode that iselectrically connected to the other pixels, a charge capturing unit thatis separated from the other pixels, and a photoelectric conversion layerthat is located between the electrode and the charge capturing unit, thephotoelectric conversion layer being continuous among the pixels,wherein the photoelectric conversion layer contains semiconductor carbonnanotubes, and one of a first substance and a second substance, thefirst substance having an electron affinity larger than that of thesemiconducting carbon nanotubes, the second substance having aionization energy smaller than that of the semiconductor carbonnanotubes.

According to an aspect of the present disclosure, an imaging deviceincluding carbon nanotubes as a photoelectric conversion material isprovided.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a circuit of an imaging deviceaccording to a first embodiment of the present disclosure;

FIG. 2 schematically illustrates a cross-section of an example of adevice configuration in each unit pixel cell of the imaging deviceillustrated in FIG. 1 ;

FIG. 3A schematically illustrates a cross-section of an example of theconfiguration of a photoelectric conversion unit of the imaging deviceillustrated in FIG. 1 ;

FIG. 3B schematically illustrates a cross-section of another example ofthe configuration of the photoelectric conversion unit of the imagingdevice illustrated in FIG. 1 ;

FIG. 4A schematically illustrates an example of the structure of acarbon nanotube;

FIG. 4B schematically illustrates the chirality of a carbon nanotube;

FIG. 5 illustrates the relationship between the chirality of a carbonnanotube and first and second resonant wavelengths;

FIG. 6A schematically illustrates the magnitude relationship between theelectron affinity of a semiconducting carbon nanotube and the electronaffinity of a charge separation material in the imaging device accordingto the first embodiment of the present disclosure;

FIG. 6B schematically illustrates the magnitude relationship between theionization energy of the semiconducting carbon nanotube and theionization energy of the charge separation material in the imagingdevice according to the first embodiment;

FIG. 7 schematically illustrates charge movement in the photoelectricconversion unit of the imaging device according to the first embodimentof the present disclosure;

FIG. 8 schematically illustrates charge movement in a carbon nanotube;

FIG. 9 schematically illustrates a cross-section of an example of theconfiguration of a photoelectric conversion unit of an imaging deviceaccording to a second embodiment; and

FIG. 10 schematically illustrates charge movement in the photoelectricconversion unit of the imaging device according to the secondembodiment.

DETAILED DESCRIPTION

There is a demand for imaging devices having sensitivity to thenear-infrared region to be used for in-vehicle cameras, monitoringcameras, and the like. Therefore, it is considered that organicsemiconductor materials having sensitivity to the near-infrared regionare used as photoelectric conversion materials for photoelectricconversion elements. The inventors have focused on carbon nanotubes as amaterial that absorbs light in the near-infrared region.

Molecules of carbon nanotubes typically have a tubular shape with alength of about several tens of nanometers to several millimeters andhave unique characteristics that are not seen in a variety of organicmaterials and inorganic materials of the related art. In particular, itis known that the electron and hole mobility of carbon nanotubes isextremely high.

By using such characteristics of carbon nanotubes, the inventors havearrived at a novel imaging device that enables high-speed operation. Anoverview of an aspect of the present disclosure is as follows.

Item 1

An imaging device comprising: pixels arranged one-dimensionally ortwo-dimensionally, each of the pixels including an electrode that iselectrically connected to the other pixels, a charge capturing unit thatis separated from the other pixels, and a photoelectric conversion layerthat is located between the electrode and the charge capturing unit, thephotoelectric conversion layer being continuous among the pixels,wherein the photoelectric conversion layer contains semiconductor carbonnanotubes, and one of a first substance and a second substance, thefirst substance having an electron affinity larger than that of thesemiconducting carbon nanotubes, the second substance having aionization energy smaller than that of the semiconductor carbonnanotubes. According to this configuration, an imaging device containingcarbon nanotubes as a photoelectric conversion material is realized. Bycontaining the substance having a larger electron affinity than thesemiconducting carbon nanotubes or the substance having a smallerionization energy than the semiconducting carbon nanotubes, the imagingdevice can suppress mixing of detected charges among pixels and canacquire high-definition images or can have a minute pixel size.

Item 2

The imaging device according to Item 1, wherein the photoelectricconversion layer contains the first substance, and the charge capturingunit captures negative charges. According to this configuration, thenegative charges can be detected as signal charges.

Item 3

The imaging device according to Item 1, wherein the photoelectricconversion layer contains the second substance, and the charge capturingunit captures positive charges. According to this configuration, thepositive charges can be detected as signal charges.

Item 4

The imaging device according to any of Items 1 to 3, further comprising:a semiconductor substrate that supports the photoelectric conversionlayer, wherein each of the pixels is provided on the semiconductorsubstrate and further includes a charge detecting transistor that iselectrically connected to the charge capturing unit. According to thisconfiguration, a stacked imaging device can be realized.

Item 5

The imaging device according to any of Items 1 to 4, wherein thesemiconductor carbon nanotubes and the one of the first substance andthe second substance are dispersed to each other in the photoelectricconversion layer. According to this configuration, the mixing of chargesamong pixels can be more reliably suppressed.

Item 6

The imaging device according to Item 4, wherein the charge capturingunit is an impurity diffusion region in the semiconductor substrate.

Item 7

The imaging device according to Item 4, wherein the charge capturingunit is an electrode located above the semiconductor substrate.

Item 8

The imaging device according to Item 2, wherein the first substance is amolecule having a fullerene skeleton According to this configuration,the negative charges can be detected as signal charges.

Item 9

The imaging device according to Item 3, further comprising a voltagesupply circuit that is electrically connected to the electrode and thatgenerates a bias voltage that is set in such a manner that the potentialof the charge capturing unit is higher than the potential of theelectrode.

Item 10

The imaging device according to Item 2, further comprising a voltagesupply circuit that is electrically connected to the electrode and thatgenerates a bias voltage that is set in such a manner that the potentialof the charge capturing unit is lower than the potential of theelectrode.

Item 11

An imaging device comprising a plurality of pixels that are arrayedone-dimensionally or two-dimensionally. Each of the pixels includes anelectrode that is electrically connected to other pixels, a chargecapturing unit that is separated from the other pixels, and aphotoelectric conversion layer that is located between the electrode andthe charge capturing unit and that contains a semiconducting carbonnanotubes and one of a first substance and a second substance, the firstsubstance having a larger electron affinity than the semiconductingcarbon nanotubes, the second substance having a smaller ionizationenergy than the semiconducting carbon nanotubes, wherein at least someof the semiconducting carbon nanotubes are electrically connected to thecharge capturing unit. This configuration contains semiconducting carbonnanotubes having high carrier mobility, and charges that move throughthe semiconducting carbon nanotubes are captured by the electrode.Accordingly, the imaging device can capture images with a high-speedoperation and at a high frame rate.

Item 12

The imaging device according to Item 11, wherein the photoelectricconversion layer contains the first substance, and the charge capturingunit captures positive charges. According to this configuration, thepositive charges can be detected as signal charges.

Item 13

The imaging device according to Item 11, wherein the photoelectricconversion layer contains the second substance, and the charge capturingunit captures negative charges. According to this configuration, thenegative charges can be detected as signal charges.

Item 14

The imaging device according to any of Items 11 to 13, furthercomprising a semiconductor substrate that supports the photoelectricconversion layer, wherein each of the pixels is provided on thesemiconductor substrate and further includes a charge detectingtransistor that is electrically connected to the charge capturing unit.According to this configuration, a stacked imaging device can berealized.

Item 15

The imaging device according to Item 14, wherein the charge capturingunit is an impurity diffused region that is formed in the semiconductorsubstrate.

Item 16

The imaging device according to Item 14, wherein the charge capturingunit is an electrode that is located on the semiconductor substrate.

Item 17

The imaging device according to Item 12, wherein the first substance isa molecule having a fullerene skeleton. According to this configuration,the positive charges can be detected as signal charges.

Item 18

The imaging device according to Item 12, further comprising a voltagesupply circuit that is electrically connected to the electrode and thatgenerates a bias voltage that is set in such a manner that the potentialof the charge capturing unit is lower than the potential of theelectrode.

Item 19

The imaging device according to Item 13, further comprising a voltagesupply circuit that is electrically connected to the electrode and thatgenerates a bias voltage that is set in such a manner that the potentialof the charge capturing unit is higher than the potential of theelectrode.

Embodiments of the present disclosure will be described below in detail.The following embodiments are general or specific examples. Values,shapes, materials, components, arrangements and connection modes of thecomponents, steps, order of the steps, and the like described in thefollowing embodiments are examples and do not limit the presentdisclosure. Various aspects described herein may be combined with oneanother insofar as the combinations do not cause contradictions. Inaddition, among the components described in the following embodiments,components that are not described in independent claims, which definebroadest concepts, are described as arbitrary components. In thefollowing description, components having substantially the samefunctions are given the same reference numerals, and redundantdescription thereof might be omitted.

First Embodiment

1. General Configuration of Imaging Device

An overview of the configuration of an imaging device according to thepresent disclosure will be described with reference to FIG. 1 . FIG. 1illustrates an example of a circuit configuration of an imaging deviceaccording to an embodiment of the present disclosure. An imaging device100 illustrated in FIG. 1 includes a plurality of unit pixel cells 20and a peripheral circuit. The peripheral circuit includes a voltagesupply circuit 10 that supplies a predetermined voltage to each of theunit pixel cells 20.

The unit pixel cells 20 are arranged one-dimensionally ortwo-dimensionally on a semiconductor substrate, thereby forming aphotosensitive region (pixel region). In the configuration illustratedin FIG. 1 , the unit pixel cells 20 are arrayed along a row directionand a column direction. The row direction and the column directionherein respectively mean the direction in which rows extend and thedirection in which columns extend. That is, the perpendicular directionon FIG. 1 is the column direction, and the horizontal direction is therow direction. In FIG. 1 , four unit pixel cells 20 that are arranged ina 2×2 matrix are illustrated. The number of unit pixel cells 20illustrated in FIG. 1 is merely an example for description, and thenumber of unit pixel cells 20 is not limited to four. If the unit pixelcells 20 are arranged one-dimensionally, the imaging device 100 is aline sensor.

Each of the unit pixel cells 20 includes a photoelectric conversion unit30 and a signal detection circuit 40 that detects signals generated bythe photoelectric conversion unit 30. The photoelectric conversion unit30 includes a pixel electrode (charge capturing unit) 32, a counterelectrode 38, and a photoelectric conversion layer 39 disposed betweenthe pixel electrode 32 and the counter electrode 38. As in theillustration, the counter electrode (electrode) 38 is connected to thevoltage supply circuit 10 via a storage control line 22. At the time ofthe operation of the imaging device 100, a predetermined bias voltage isapplied to the counter electrode 38 through the storage control line 22.

As will be described later in detail, the photoelectric conversion layer39 contains semiconducting carbon nanotubes and is configured in such amanner that the pixel electrode 32 captures either holes (positivecharges) or electrons (negative charges) as signal charges amonghole-electron pairs generated through photoelectric conversion. Thepotential of the counter electrode 38 is controlled by using the biasvoltage generated by the voltage supply circuit 10, and thereby eitherholes or electrons can be captured by the pixel electrode 32. Forexample, in a case of using holes as signal charges, in order to set thepotential of the counter electrode 38 higher than the potential of thepixel electrode 32, a voltage of about 10 V, for example, is applied tothe storage control line 22.

In the configuration illustrated in FIG. 1 , the signal detectioncircuit 40 includes an amplifier transistor (charge detectingtransistor) 42, an address transistor (row selecting transistor) 44, anda reset transistor 46. Typically, the amplifier transistor 42 and theaddress transistor 44 are field-effect transistors (FETs) formed on asemiconductor substrate. Unless otherwise specified, an example of usingn-channel MOS transistors will be described below. Note that the“semiconductor substrate” herein is not limited to a substrate that isentirely formed of a semiconductor, but may be an insulating substrate,for example, having a semiconductor layer on a surface on which aphotosensitive region is formed. Examples of the semiconductor substrateinclude a p-type silicon substrate.

As in the illustration, one (typically, a source) of an input terminaland an output terminal of the amplifier transistor 42 is connected toone (typically, a drain) of an input terminal and an output terminal ofthe address transistor 44. A control terminal (gate) of the amplifiertransistor 42 is electrically connected to the pixel electrode 32 of thephotoelectric conversion unit 30. Signal charges (e.g., holes) collectedby the pixel electrode 32 are stored in a charge storing node (alsoreferred to as “floating diffusion node”) 41 between the pixel electrode32 and the gate of the amplifier transistor 42.

A voltage in accordance with the signal charges stored at the chargestoring node 41 is applied to the gate of the amplifier transistor 42.The amplifier transistor 42 amplifies this voltage. That is, theamplifier transistor 42 amplifies the signal generated by thephotoelectric conversion unit 30. The voltage amplified by the amplifiertransistor 42 is selectively read as a signal voltage through theaddress transistor 44.

One of the source and the drain of the reset transistor 46 is connectedto the charge storing node 41 and is electrically connected to the pixelelectrode 32.

The reset transistor 46 resets the signal charges stored at the chargestoring node 41. In other words, the reset transistor 46 resets thepotential of the gate of the amplifier transistor 42 and the potentialof the pixel electrode 32.

As in the illustration, the imaging device 100 includes a power sourceline 23, vertical signal lines 24, address signal lines 25, and resetsignal lines 26. These lines are connected to each of the unit pixelcells 20. The power source line (source-follower power source) 23 isconnected to one of the source and the drain of the amplifier transistor42 and supplies a predetermined power source voltage to each of the unitpixel cells 20. Of the source and the drain of the address transistor44, a vertical signal line 24 is connected to either one that is notconnected to the source or the drain of the amplifier transistor 42. Anaddress signal line 25 is connected to the gate electrode of the addresstransistor 44. A reset signal line 26 is connected to the gate of thereset transistor 46.

The peripheral circuit of the imaging device 100 includes a verticalscanning circuit (also referred to as “row scanning circuit”) 52, ahorizontal signal-reading circuit (also referred to as “column scanningcircuit”) 54, a plurality of column signal processing circuits (alsoreferred to as “row signal storing circuits”) 56, a plurality of loadcircuits 58, and a plurality of inverting amplifiers 59. The columnsignal processing circuits 56, the load circuits 58, and the invertingamplifiers 59 are provided to correspond to the respective columns ofthe plurality of unit pixel cells 20 arrayed in the row direction andthe column direction. Each of the column signal processing circuits 56is electrically connected to unit pixel cells 20 arranged in acorresponding column via a vertical signal line 24 corresponding to eachcolumn of the plurality of unit pixel cells 20. The plurality of columnsignal processing circuits 56 are electrically connected to thehorizontal signal-scanning circuit 54. Each of the load circuits 58 iselectrically connected to a corresponding one of the vertical signallines 24, and the load circuits 58 and the amplifier transistors 42 formsource-follower circuits.

The vertical scanning circuit 52 is connected to the address signallines 25 and the reset signal lines 26. The vertical scanning circuit 52applies a row selection signal for controlling ON and OFF of the addresstransistors 44 to the gates of the address transistors 44 through theaddress signal lines 25. By the row selection signal being sent througha corresponding one of the address signal lines 25, a reading target rowis scanned and selected. A signal voltage is read from unit pixel cells20 in the selected row to a corresponding one of the vertical signallines 24. In addition, the vertical scanning circuit 52 applies a resetsignal for controlling ON and OFF of the reset transistors 46 to thegates of the reset transistors 46 through the reset signal lines 26. Bythe row selection signal being sent through a corresponding one of thereset signal lines 26, a reset-operation target row of the unit pixelcells 20 is selected. In this manner, the vertical scanning circuit 52selects a plurality of unit pixel cells 20 in units of row and reads thesignal voltage and resets the potentials of the pixel electrodes 32.

The signal voltage read from the unit pixel cells 20 selected by thevertical scanning circuit 52 is transferred to a column signalprocessing circuit 56 through a vertical signal line 24. The columnsignal processing circuit 56 perform noise-cancel signal processingtypified by correlated double sampling, analog-digital conversion (ADconversion), and the like. The horizontal signal-scanning circuit 54reads signals sequentially from the plurality of column signalprocessing circuits 56 to a horizontal common signal line (notillustrated).

Note that the vertical scanning circuit 52 may partly include theabove-described voltage supply circuit 10. Alternatively, the voltagesupply circuit 10 may be electrically connected to the vertical scanningcircuit 52. In other words, a bias voltage may be applied to the counterelectrodes 38 through the vertical scanning circuit 52.

In the configuration illustrated in FIG. 1 , the plurality of invertingamplifiers 59 are provided to correspond to the respective columns. Anegative input terminal of each of the inverting amplifiers 59 isconnected to a corresponding one of the vertical signal lines 24. Anoutput terminal of each of the inverting amplifiers 59 is connected tounit pixel cells 20 in a corresponding column via a feedback line 27provided to correspond to each column.

As in the illustration, of the source and the drain of each resettransistor 46, the feedback line 27 is connected to either one (drain)that is not connected to the charge storing node 41. Thus, while anaddress transistor 44 and a reset transistor 46 are in a conductivestate, the negative terminal of the inverting amplifier 59 receives theoutput of the address transistor 44. On the other hand, a referencevoltage used for resetting is applied to a positive input terminal ofthe inverting amplifier 59 from a power source, which is notillustrated. The inverting amplifier 59 performs a feedback operation sothat the gate voltage of the amplifier transistor 42 is set at apredetermined feedback voltage. The feedback voltage is the outputvoltage of the inverting amplifier 59. The output voltage of theinverting amplifier 59 is, for example, 0 V or a positive voltage around0 V. The inverting amplifier 59 may also be referred to as a “feedbackamplifier”.

2. Device Configuration of Imaging Device

FIG. 2 schematically illustrates a cross-section of a deviceconfiguration of each unit pixel cell 20 of the imaging device 100according to the first embodiment of the present disclosure. In theconfiguration illustrated in FIG. 2 , the unit pixel cell 20 includes asemiconductor substrate 62 (e.g., a silicon substrate) that supports thephotoelectric conversion unit 30. As in the illustration, thephotoelectric conversion unit 30 is disposed above the semiconductorsubstrate 62. In this example, interlayer insulating layers 63A, 63B,and 63C are stacked on the semiconductor substrate 62, and a stack ofthe pixel electrode 32, the photoelectric conversion layer 39, and thecounter electrode 38 is disposed on the interlayer insulating layer 63C.The pixel electrode 32 is divided for each pixel, and the pixelelectrodes 32 are formed to be spatially separate per two adjacent unitpixel cells 20, and thereby two adjacent pixel electrodes 32 areelectrically separated from each other. In addition, the photoelectricconversion layer 39 and the counter electrode 38 are formed to extendover a plurality of unit pixel cells 20. The counter electrode 38 isformed of, for example, a metal oxide such as ITO or ZnO, few-layergraphene, a metal nanowire, or the like.

The amplifier transistor 42, the address transistor 44, and the resettransistor 46 are formed on the semiconductor substrate 62.

The amplifier transistor 42 includes impurity regions 62 a and 62 bformed in the semiconductor substrate 62, a gate insulating layer 42 glocated on the semiconductor substrate 62, and a gate electrode 42 elocated on the gate insulating layer 42 g. The impurity regions (n-typeimpurity regions) 62 a and 62 b serve as the drain and the source of theamplifier transistor 42.

The address transistor 44 includes impurity regions 62 a and 62 c formedin the semiconductor substrate 62, a gate insulating layer 44 g locatedon the semiconductor substrate 62, and a gate electrode 44 e located onthe gate insulating layer 44 g. The impurity regions (n-type impurityregions) 62 a and 62 c serve as the drain and the source of the addresstransistor 44. In this example, by sharing the impurity region 62 abetween the amplifier transistor 42 and the address transistor 44, thesource (or the drain) of the amplifier transistor 42 is electricallyconnected to the drain (or the source) of the address transistor 44.

The reset transistor 46 includes impurity regions 62 d and 62 e formedin the semiconductor substrate 62, a gate insulating layer 46 g locatedon the semiconductor substrate 62, and a gate electrode 46 e located onthe gate insulating layer 46 g. The impurity regions (n-type impurityregions) 62 d and 62 e serve as the drain and the source of the resettransistor 46.

In the semiconductor substrate 62, element separation regions 62 s areprovided between adjacent unit pixel cells 20 and between the amplifiertransistor 42 and the reset transistor 46. The adjacent unit pixel cells20 are electrically separated from each other by the element separationregions 62 s. In addition, by the element separation regions 62 s beingprovided between the adjacent unit pixel cells 20, leakage of signalcharges stored at the charge storing node 41 is suppressed.

In the interlayer insulating layer 63A, a contact plug 65A connected tothe impurity region 62 d of the reset transistor 46, a contact plug 65Bconnected to the gate electrode 42 e of the amplifier transistor 42, anda wiring 66A connecting the contact plug 65A and the contact plug 65B toeach other are formed. Thus, the n-type impurity region 62 d (e.g., thedrain) of the reset transistor 46 is electrically connected to the gateelectrode 42 e of the amplifier transistor 42. In the configurationillustrated in FIG. 2 , a plug 67A and a wiring 68A are further formedin the interlayer insulating layer 63A. In addition, a plug 67B and awiring 68B are formed in the interlayer insulating layer 63B, and a plug67C is formed in the interlayer insulating layer 63C. Thus, the wiring66A is electrically connected to the pixel electrode 32. The contactplug 65A, the contact plug 65B, the wiring 66A, the plug 67A, the wiring68A, the plug 67B, the wiring 68B, and the plug 67C are typically formedof metals.

In the configuration illustrated in FIG. 2 , a protective layer 72 isprovided on the counter electrode 38. The protective layer 72 is not asubstrate disposed to support the photoelectric conversion unit 30. Asillustrated in FIG. 2 , microlenses may be disposed on the protectivelayer 72.

3. Configuration of Photoelectric Conversion Layer

Next, the photoelectric conversion layer 39 in the photoelectricconversion unit 30 will be described in detail. FIG. 3A schematicallyillustrates a cross-section of the photoelectric conversion unit 30 in aunit pixel cell 20. In each unit pixel cell 20, the photoelectricconversion unit 30 includes the counter electrode 38, the pixelelectrode 32, and the photoelectric conversion layer 39. Thephotoelectric conversion layer 39 is sandwiched between the counterelectrode 38 and the pixel electrode 32.

In order to suppress the generation of leak current, the photoelectricconversion layer 39 is not separate for each pixel. That is, thephotoelectric conversion layer 39 is formed to extend over a pluralityof unit pixel cells 20. The photoelectric conversion layer 39 containssemiconducting carbon nanotubes 105 and a charge separation material106.

The carbon nanotubes will be described in detail. As illustrated in FIG.4A, each carbon nanotube has a structure obtained by wrapping, into acylinder, a single-layer graphite sheet called graphene on which carbonatoms are disposed at the vertexes of hexagons. As carbon nanotubes,there are single-walled carbon nanotubes and multi-walled carbonnanotubes. In the imaging device according to the present disclosure,either the single-walled carbon nanotubes or the multi-walled carbonnanotubes may be used. Note that in order to control the physicalproperties of the carbon nanotubes in accordance with chirality, whichwill be described later, the single-walled carbon nanotubes aredesirably used.

In a case of wrapping graphene into a cylinder, carbon nanotubes havingdifferent diameters and different carbon arrangements can be formeddepending on the thickness of the cylinder or the shift amount betweenboth ends of a graphene sheet in the longitudinal direction. Asillustrated in FIG. 4B, a rectangle having principal lattice vectors a₁and a₂ and two sides OP and OQ in a two-dimensional arrangement ofcarbon atoms at vertexes of hexagons will be considered. With point Obeing the origin, point P is represented by a vector (6,3). Thisrectangle is wrapped in such a manner that point P corresponds to pointO, and a resultant carbon nanotube is referred to as a carbon nanotubewith a chirality (6,3).

By specifying the chirality, the positions of carbon atoms in the carbonnanotube are determined. Therefore, in accordance with chirality, theelectronic structure of the carbon nanotube is determined, and thephysical properties of the carbon nanotube are determined. That is, agiven carbon nanotube is specified by a chiral vector represented byC=na₁+ma₂ (where n and m are integers). At this time, (n, |m|) is calleda chiral index or chirality. Note that |m| represents the absolute valueof m. In this representation of chirality, right-handed wrapping andleft-handed wrapping of the carbon nanotube are not distinguished.

There are carbon nanotubes having metallic properties and carbonnanotubes having semiconducting properties. Carbon nanotubes can havemetallic properties or semiconducting properties depending on chirality.In the imaging device according to the present disclosure,semiconducting carbon nanotubes are used. That is, the carbon nanotubesare used not as simple electric conductors, but as light detectors,specifically, generators of hole-electron pairs through photoelectricconversion. It is desirable that the ratio of metallic carbon nanotubesin the photoelectric conversion layer 39 be small. It is more desirablethat the metallic carbon nanotubes be excluded. This is becausehole-electron pairs that have been generated disappear immediately inthe metallic carbon nanotubes.

The carrier mobility of the semiconducting carbon nanotubes is notlargely dependent on chirality. Typically, the hole mobility and theelectron mobility of the semiconducting carbon nanotubes are eachseveral tens of thousands of square centimeters per volt-second, whichis ten times or more higher the electron mobility of silicon.

The resonant wavelength, that is, the absorption wavelength, of thecarbon nanotubes is dependent on chirality. Thus, by changing chirality,the absorption wavelength of the carbon nanotubes can be adjusted.

FIG. 5 illustrates the relationship between the chirality of asemiconducting carbon nanotube and first and second resonantwavelengths. The first resonant wavelength refers to the longestwavelength at which resonance occurs, and the second resonant wavelengthrefers to the second longest wavelength at which resonance occurs.Although not illustrated, the semiconducting carbon nanotube may haveother resonant wavelengths.

It is found from FIG. 5 that wavelengths that can be resonantly absorbedare distributed in the visible region to the far-infrared region. Inparticular, a semiconducting carbon nanotube having a diameter of about1 nm, which are manufactured with ease, have resonant wavelengths thatare distributed from the visible region to the near-infrared regionwhere the wavelength is about 1.6 μm.

For example, a semiconducting carbon nanotube having a chirality (8,0)has a resonant wavelength of about 780 nm. In addition, a semiconductingcarbon nanotube having a chirality (7,2) has a resonant wavelength ofabout 820 nm. Deoxyhemoglobins absorbs light more than oxyhemoglobins ata wavelength of 780 nm, and oxyhemoglobins have higher absorption thandeoxyhemoglobins at a wavelength of 820 nm. Thus, by using thesemiconducting carbon nanotubes having such chiralities for aphotoelectric conversion layer, an imaging device that enables imagingof the oxygen concentration in blood can be realized.

For example, semiconducting carbon nanotubes having chiralities (12,4),(10,6), (13,0), and the like resonantly absorb near-infrared light ofabout 1.4 μm. This corresponds to a wavelength band that is deficit insunlight. Thus, by using the semiconducting carbon nanotubes having suchchiralities for a photoelectric conversion layer and by usingillumination light having a wavelength of about 1.4 μm, an imagingdevice that enables image capturing without being influenced by sunlightcan be realized.

In addition, a semiconducting carbon nanotube having a chirality (8,7)has a resonant wavelength of about 1.3 μm. This is close to thewavelength band that is deficit in sunlight, but sunlight does notattenuate very much at this wavelength. Thus, by using semiconductingcarbon nanotubes having such a chirality for a photoelectric conversionlayer, an imaging device that enables image capturing using sunlightnear the wavelength band that is deficit in sunlight can be realized.

Furthermore, each semiconducting carbon nanotube has several resonantwavelengths, and the resonant wavelengths may be present in thenear-infrared region and in the visible region. For example, resonanceof a semiconducting carbon nanotube having a chirality (10,6) is presentat about 750 nm in a visible region in addition to about 1.4 μm in theinfrared region. Thus, by using semiconducting carbon nanotubes havingsuch a chirality for a photoelectric conversion layer in combinationwith an optical filter or the like, an imaging device that enablesmultispectral imaging is realized.

The photoelectric conversion layer 39 may contain the semiconductingcarbon nanotubes 105 having a predetermined one chirality or may containthe semiconducting carbon nanotubes 105 having two or more differentchiralities. By the semiconducting carbon nanotubes 105 having two ormore different chiralities being contained, the wavelength band of lightthat can be detected by the imaging device can be expanded.

Semiconducting carbon nanotubes having resonant wavelengths in thevisible and near-infrared region have a diameter of about 1 nm.

Next, the charge separation material 106 will be described in detail.The charge separation material 106 can have a semiconducting property, asemi-metallic property, or the like. Among hole-electron pairs generatedby the semiconducting carbon nanotubes 105 receiving light, the chargeseparation material 106 separates holes or electrons from thesemiconducting carbon nanotubes 105. In this embodiment, the chargeseparation material 106 separates, from the semiconducting carbonnanotubes 105, holes or electrons that are not to be detected as signalcharges.

For example, if holes are used as signal charges, the voltage supplycircuit 10 supplies the counter electrode 38 with a bias voltage that isset in such a manner that the potential of the pixel electrode 32 islower than the potential of the counter electrode 38. In this case, asubstance having a larger electron affinity than the semiconductingcarbon nanotubes 105 can be used as the charge separation material 106.

As illustrated in FIG. 6A, the electron affinity is defined as theenergy difference from the bottom of a conduction band 110 to a vacuumlevel 113 in semiconductors. If a semiconductor is an organicsemiconductor, the electron affinity is the energy difference betweenthe LUMO level and the vacuum level. More typically, the electronaffinity is the maximum energy difference between an energy level atwhich one extra electron can be received in a neutral state and thevacuum level. If the charge separation material 106 is a substancehaving a larger electron affinity than the semiconducting carbonnanotubes 105, among hole-electron pairs generated in the semiconductingcarbon nanotubes 105, electrons move to the conduction band 110 of thecharge separation material 106, which is at a lower energy level. Thus,holes remain in a valence band 111 of the semiconducting carbonnanotubes 105.

Since a bias voltage that is set in such a manner that the potential ofthe pixel electrode 32 is lower than the potential of the counterelectrode 38 is supplied, holes move from the semiconducting carbonnanotubes 105 to the pixel electrode 32.

The electron affinity of semiconducting carbon nanotubes having resonantwavelengths in the visible and near-infrared region is about 3 eV to 4eV. Examples of the substance having a larger electron affinity than thesemiconducting carbon nanotubes 105 typically include fullerene andfullerene derivatives. Fullerene and fullerene derivatives are referredto as molecules having fullerene skeletons. For example, C60 has anelectron affinity of about 4 eV. Besides, C70, C80, and compoundsobtained by introducing modifying groups to such fullerene skeletons,such as phenyl-C61-butyric acid methyl ester (PCBM), indene-C60bisadduct (ICBA), and indene-C60 monoadduct (ICMA), also have electronaffinity of about 3.7 eV to 3.8 eV.

For example, if electrons are used as signal charges, the voltage supplycircuit 10 supplies the counter electrode 38 with a bias voltage that isset in such a manner that the potential of the pixel electrode 32 ishigher than the potential of the counter electrode 38. In this case, asubstance having a smaller ionization energy than the semiconductingcarbon nanotubes 105 can be used as the charge separation material 106.

As illustrated in FIG. 6B, the ionization energy is defined as theenergy difference from the bottom of the valence band 111 to the vacuumlevel 113 in semiconductors. If a semiconductor is an organicsemiconductor, the ionization energy is the energy difference betweenthe HOMO level and the vacuum level. More typically, the ionizationenergy is the minimum energy necessary to elevate one electron from theneutral state to the vacuum level. If the charge separation material 106is a substance having a smaller ionization energy than thesemiconducting carbon nanotubes 105, the charge separation material 106is more stabilized by becoming a positive ion. Thus, among hole-electronpairs generated in the semiconducting carbon nanotubes 105, holes moveto the valence band 111 of the charge separation material 106. Thus,electrons remain in the conduction band 110 of the semiconducting carbonnanotubes 105.

Since a bias voltage that is set in such a manner that the potential ofthe pixel electrode 32 is higher than the potential of the counterelectrode 38 is supplied to the counter electrode 38, electrons movefrom the semiconducting carbon nanotubes 105 to the pixel electrode 32.

The ionization energy of semiconducting carbon nanotubes having resonantwavelengths in the visible and near-infrared region is about 4 eV to 5eV. Examples of a substance having a smaller ionization energy than thesemiconducting carbon nanotubes 105 includepoly(3-dodecylthiophene-2,5-diyl) (P3DDT, ionization energy: about 4.6eV), poly(3-hexylthiophene-2,5-diyl) (P3HT, ionization energy: about 5eV), and the like.

As described above, the electronic structures of the semiconductingcarbon nanotubes 105 differ depending on chirality. Thus, the electronaffinity and ionization energy of the semiconducting carbon nanotubes105 also differ depending on chirality. Accordingly, it is desirable tocalculate the electron affinity and the ionization energy of thesemiconducting carbon nanotubes 105 in consideration of chirality toselect the charge separation material 106 so as to satisfy theabove-described relationship.

In the photoelectric conversion layer 39, both the semiconducting carbonnanotubes 105 and the charge separation material 106 may be dispersed.In this case, the distribution of the semiconducting carbon nanotubes105 and the charge separation material 106 does not have to be uniform.For example, the distribution may be biased in the thickness directionof the photoelectric conversion layer 39.

It is desirable that signal charges be movable from the semiconductingcarbon nanotubes 105 to the pixel electrode 32. In other words, it isdesirable that the semiconducting carbon nanotubes 105 be electricallyconnected to the pixel electrode 32. For example, some of thesemiconducting carbon nanotubes 105 may be in direct contact with thepixel electrode 32.

As illustrated in FIG. 3B, the photoelectric conversion unit 30 mayfurther include a hole blocking layer 103 and an electron blocking layer101. These layers are disposed at positions where the movement of holesand electrons is desirably suppressed. For example, if the pixelelectrode 32 captures electrons, the hole blocking layer 103 is providedbetween the pixel electrode 32 and the photoelectric conversion layer39, and the electron blocking layer 101 is provided between the counterelectrode 38 and the photoelectric conversion layer 39.

The hole blocking layer 103 transports negative charges generated in thephotoelectric conversion layer 39 to the pixel electrode 32 and preventspositive charges from entering the photoelectric conversion layer 39from the pixel electrode 32. It is desirable to provide the holeblocking layer 103 in order to suppress current that flows even whenlight is not illuminated, which is so-called dark current. For example,bathocuproine (BCP) can be used for the hole blocking layer 103.

The electron blocking layer 101 has a role of transporting positivecharges generated in the photoelectric conversion layer 39 to thecounter electrode 38 and prevents negative charges from entering thephotoelectric conversion layer 39 from the counter electrode 38. It isdesirable to provide the electron blocking layer 101 in order tosuppress dark current. For example,poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) canbe used for the electron blocking layer 101.

The hole blocking layer 103 and the electron blocking layer 101 are bothelectrically conductive in order to transport electrons (negativecharges) and holes (positive charges). Thus, if the hole blocking layer103 is provided between the pixel electrode 32 and the photoelectricconversion layer 39, by some of the semiconducting carbon nanotubes 105being in contact with the hole blocking layer 103, the semiconductingcarbon nanotubes 105 are electrically connected to the pixel electrode32 through the hole blocking layer 103. In addition, if the electronblocking layer 101 is provided between the pixel electrode 32 and thephotoelectric conversion layer 39, by some of the semiconducting carbonnanotubes 105 being in contact with the electron blocking layer 101, thesemiconducting carbon nanotubes 105 are electrically connected to thepixel electrode 32 through the electron blocking layer 101.

If the hole blocking layer 103 is provided between the photoelectricconversion layer 39 and the pixel electrode 32, the hole blocking layer103 may be provided in any manner as long as signal charges can movebetween the semiconducting carbon nanotubes 105 and the hole blockinglayer 103 and between the hole blocking layer 103 and the pixelelectrode 32. Some of the semiconducting carbon nanotube 105 may be indirect contact with the hole blocking layer 103.

The semiconducting carbon nanotubes 105 do not have to be in directcontact with the pixel electrode 32 or the hole blocking layer 103 aslong as signal charges can move. This is because the signal charges canmove through another charge transport material or by the tunnelingphenomenon or the like. Note that the charge transportation through theother charge transport material and by the tunneling phenomenon istypically slower than the charge transportation through thesemiconducting carbon nanotubes 105. Accordingly, it is desirable thatas many semiconducting carbon nanotubes 105 as possible be in directcontact with the pixel electrode 32 or the hole blocking layer 103, orit is desirable that some of the semiconducting carbon nanotube 105 bedistributed to the vicinity of the pixel electrode 32 or the holeblocking layer 103.

Note that if holes are captured as signal charges, the electron blockinglayer 101 is provided between the photoelectric conversion layer 39 andthe pixel electrode 32. In this case, by replacing the hole blockinglayer 103 with the electron blocking layer 101, the arrangement of thesemiconducting carbon nanotubes 105 can be described in the same manner.

In the photoelectric conversion layer 39, it is desirable that as manymolecules of the semiconducting carbon nanotubes 105 as possible be incontact with the pixel electrode 32 or a larger number of molecules ofthe semiconducting carbon nanotubes 105 be distributed closer to thepixel electrode 32. Thus, the density of the semiconducting carbonnanotubes 105 in the thickness direction of the photoelectric conversionlayer 39 may be low on the counter electrode 38 side and may be high onthe pixel electrode 32 side. In this case, it is desirable that thedensity of the charge separation material 106 in the thickness directionof the photoelectric conversion layer 39 be high on the counterelectrode 38 side and be low on the pixel electrode 32 side.

As illustrated in FIG. 3A, the photoelectric conversion layer 39 mayinclude a layer 39 a containing only the charge separation material 106and a layer 39 b containing only the semiconducting carbon nanotubes105. In this case, the layer 39 b containing only the semiconductingcarbon nanotubes 105 is in contact with the pixel electrode 32, and thelayer 39 a containing only the charge separation material 106 is incontact with the counter electrode 38.

The thickness of the photoelectric conversion layer 39 is, for example,greater than or equal to several tens of nanometers and less than orequal to several hundreds of nanometers. In addition, the photoelectricconversion layer 39 may further include an n-type semiconductor or ap-type semiconductor.

4. Charge Movement in Photoelectric Conversion Layer 39

The detection of incident light in the photoelectric conversion layer 39will be described with reference to FIG. 7 . FIG. 7 schematicallyillustrates the positional relationship among the semiconducting carbonnanotubes 105 and the charge separation material 106 in thephotoelectric conversion layer 39, the pixel electrode 32, and thecounter electrode 38. In FIG. 7 , only a semiconducting carbon nanotube105 of interest and a charge separation material 106 of interest aredenoted by solid lines. In the following example, electrons are used assignal charges. In addition, the charge separation material 106 is asubstance having a smaller ionization energy than the semiconductingcarbon nanotube 105. In FIG. 7 , a bias voltage is supplied to thecounter electrode 38 in such a manner that the potential of the pixelelectrode 32 is higher than the potential of the counter electrode 38.Thus, an electric field is generated in the direction indicated by anarrow 502.

Upon absorption of a photon (not illustrated) by the semiconductingcarbon nanotube 105, a positive charge (hole) h and a negative charge(electron) e are generated at a position 501 that is substantially thesame as the position of the absorption. The generated positive charge hand negative charge e have attractive forces with respect to each other,creating a state called an exciton. Excitons are electrically neutral ina macroscopic view and thus only move gradually so as to be dispersed,the positions of the excitons not being influenced by the appliedelectric field.

Upon an exciton arriving at a position close to the charge separationmaterial 106, the charges of the exciton are separated. Specifically,the charge separation material 106 has a smaller ionization energy thanthe semiconducting carbon nanotube 105, and thus, the energy is morestabilized if the charge separation material 106 receives the positivecharge h than if the semiconducting carbon nanotube 105 keeps thepositive charge h. Therefore, the charge separation material 106separates the positive charge h. Accordingly, the positive charge movesto one of the molecules of the charge separation material 106, and thenegative charge e remains in the semiconducting carbon nanotube 105.

The charges are separated at the position where the semiconductingcarbon nanotube 105 is close to the charge separation material 106.Accordingly, if one or more molecules of the charge separation material106 are present within a unit pixel cell 20 of the periphery of one ofthe molecules of the semiconducting carbon nanotube 105, the charges areseparated in the pixel where the exciton has been generated. In order tocause the exciton to be generated and the charges to be separated withinthe same unit pixel cell, the ratio of the charge separation material106 contained in the photoelectric conversion layer 39 is desirablyincreased.

The negative charge e remaining in the semiconducting carbon nanotube105 and the positive charge h that has moved to the charge separationmaterial 106 are no longer neutral even in a macroscopic view.Accordingly, both the negative charge e and the positive charge h areinfluenced by the electric field indicated by the arrow 502 due to thevoltage applied between the counter electrode 38 and the pixel electrode32 and start to move.

The negative charge e remaining in the semiconducting carbon nanotube105 is influenced by the electric field indicated by the arrow 502 andmoves, within the semiconducting carbon nanotube 105, to the pixelelectrode 32 side. This movement is extremely fast. As indicated by anarrow 504, the negative charge e moves to the position where thesemiconducting carbon nanotube 105 is in contact with the pixelelectrode 32 and is captured by the pixel electrode 32. That is, in theimaging device according to the present disclosure, the chargesgenerated through photoelectric conversion are detected in an extremelyshort period.

On the other hand, the positive charge h that has moved to the one ofthe molecules of the charge separation material 106 moves to an adjacentmolecule of the charge separation material 106 by hopping conduction. Amolecule to which the positive charge h hops is determined by thedistance between molecules and the direction and strength of theelectric field. If the molecules have substantially equal distancestherebetween, a molecule in a direction that is closer to the directionparallel to the electric field is selected as the hopping destination.If the strength of the electric field is strong, even though distancesbetween molecules are different, the possibility that a molecule in adirection that is closer to the direction parallel to the electric fieldis selected as the hopping destination is higher. Accordingly, if thedistribution of distances between molecules of the charge separationmaterial 106 is not largely biased and the charge separation material106 is distributed three-dimensionally in the photoelectric conversionlayer 39, the positive charge h moves in the direction of the electricfield in a macroscopic view and is captured by the counter electrode 38,as indicated by an arrow 503. A case where the distribution of distancesbetween molecules is largely biased is, for example, a case where themolecules of the charge separation material 106 are arranged only on acertain plane.

Typically, the carrier mobility of the charge separation material 106 isconsiderably lower than the carrier mobility of the semiconductingcarbon nanotube. The movement of the positive charge h between moleculesof the charge separation material 106 by hopping is also slower than thecarrier movement in the semiconducting carbon nanotube 105.

Although an example where the negative charge e is captured by the pixelelectrode 32 has been described with reference to FIG. 7 , the casewhere the positive charge h is captured by the pixel electrode 32 can beexplained in the same manner. In this case, the charge separationmaterial 106 can be a material having a larger electron affinity thanthe semiconducting carbon nanotube 105. Thus, the charge separationmaterial 106 separates a negative charge e from an exciton. A positivecharge h in the semiconducting carbon nanotube moves to the pixelelectrode 32.

In addition, an example in which the negative charge e generated fromthe exciton is captured directly by the pixel electrode 32 from amolecule of the semiconducting carbon nanotube 105 has been describedwith reference to FIG. 7 . However, the molecule of the semiconductingcarbon nanotube 105 does not have to be in direct contact with the pixelelectrode 32. In the photoelectric conversion layer 39, if two or moresemiconducting carbon nanotubes 105 are close to each other or are incontact with each other, and if at least one of the semiconductingcarbon nanotubes 105 is in contact with the pixel electrode 32, a chargecan be captured by the pixel electrode 32. In this case, the charge of asemiconducting carbon nanotube 105 that is not in direct contact withthe pixel electrode 32 moves as follows. First, the charge moves toanother semiconducting carbon nanotube 105 that is close to or incontact with the semiconducting carbon nanotube 105 by hopping. Byrepeating this hopping, the charge moves to the semiconducting carbonnanotube 105 that is in direct contact with the pixel electrode 32. Thecharge ends up with being captured by the pixel electrode 32 from thesemiconducting carbon nanotube 105 that is in direct contact with thepixel electrode 32.

As is clear from the above description, if the charge separationmaterial 106, even in a small content, is contained in the photoelectricconversion layer 39, a hole or an electron among hole-electron pairs canbe separated from the semiconducting carbon nanotube 105. The chargeremaining in the carbon nanotube 105 moves rapidly among thesemiconducting carbon nanotubes 105 and can be captured by the electrode32. Accordingly, the imaging device according to the present disclosurecan produce the above-described effects in accordance with the contentof the charge separation material 106 in the photoelectric conversionlayer 39.

In the above manner, the imaging device according to this embodimentincludes semiconducting carbon nanotubes having a high carrier mobility,in which a pixel electrode captures a charge that moves through thesemiconducting carbon nanotubes. Thus, an imaging device that enablesimage capturing with a high-speed operation and at a high frame rate canbe realized. For example, a Time-of-Flight image sensor can be realized.In addition, carbon nanotubes have high chemical stability and aredifficult to degrade the characteristics. Accordingly, an imaging devicethat can be used in a wide temperature range and that has highdurability and high reliability can be realized. Furthermore, there arecarbon nanotubes having different wavelength-sensitivity characteristicsdepending on the difference in chirality. Therefore, it is easy todesign the wavelength-sensitivity characteristics. In addition, animaging device that enables image capturing with multiple wavelengthscan be realized.

In particular, in an organic thin film that absorbs light in thenear-infrared region, due to the movement of organic molecules,holes-electron pairs generated through photoelectric conversion may bedeactivated, or a transition level may vary. Accordingly, in an imagingdevice, noise tends to be generated in signals from charges that havebeen detected. In contrast, carbon nanotubes have a stable and rigidstructure. Accordingly, the transition level is unlikely to vary, andsignals from charges that have been detected can be obtained with a highS/N ratio.

5. Method for Fabricating Imaging Device

The imaging device 100 can be fabricated through a typical semiconductorfabrication process. In particular, if a silicon substrate is used asthe semiconductor substrate 62, the imaging device 100 can be fabricatedthrough a variety of silicon semiconductor processes.

First, as illustrated in FIG. 2 , the amplifier transistor 42, theaddress transistor 44, and the reset transistor 46 are formed on thesemiconductor substrate 62 by using a variety of semiconductorfabrication techniques. Then, the interlayer insulating layers 63A to63C, the contact plugs 65A and 65B, the wirings 66A, 68A, and 68B, theplugs 67A to 67C, and the pixel electrode 32 are formed.

Then, as necessary, the hole blocking layer 103 is formed on theinterlayer insulating layer 63C on which the pixel electrode 32 isprovided. In a case of using BCP for the hole blocking layer 103, thehole blocking layer 103 is formed by evaporation. In a case of formingthe hole blocking layer 103 with a material that can be dissolved in asolvent, a spin coating method, an inkjet method, or the like can beemployed.

Then, the photoelectric conversion layer 39 is formed. The photoelectricconversion layer 39 contains the semiconducting carbon nanotubes 105 andthe charge separation material 106. Among them, the semiconductingcarbon nanotubes 105 are difficult to form by evaporation. Therefore,the semiconducting carbon nanotubes 105 and the charge separationmaterial 106 are separately prepared and mixed together in a solvent andthen are applied on the interlayer insulating layer 63C by spin coating,doctor blading, or inkjetting. Alternatively, the semiconducting carbonnanotubes 105 and the charge separation material 106 may be separatelydispersed in different solvents and may be applied on the interlayerinsulating layer 63C. Further alternatively, the semiconducting carbonnanotubes 105 may be applied on the interlayer insulating layer 63C, andthe charge separation material 106 may be formed on the interlayerinsulating layer 63C by evaporation. If the semiconducting carbonnanotubes 105 and the charge separation material 106 are disposed on theinterlayer insulating layer 63C through different steps, thesemiconducting carbon nanotubes 105 and the charge separation material106 may be alternately provided on the interlayer insulating layer 63C,and the semiconducting carbon nanotubes 105 or the charge separationmaterial 106 may be provided at least twice in order to disperse boththe semiconducting carbon nanotubes 105 and the charge separationmaterial 106.

In order to disperse the semiconducting carbon nanotubes 105 in asolvent, a polymer or a surface-active agent may be used as asolubilizing agent. Such a solubilizing agent may remain in thephotoelectric conversion layer 39 unless having adverse effects on thecharacteristics of an image sensor. Alternatively, only the solubilizingagent may be removed after an applying step.

The semiconducting carbon nanotubes 105 can be obtained by selectingsemiconducting ones from among manufactured semiconducting carbonnanotubes. Examples of a method for manufacturing semiconducting carbonnanotubes include a HiPco method, a CoMoCAT method, an alcohol CVDmethod, and an arc discharge method. Alternatively, such a method may beemployed after performing a chirality selecting operation in order toincrease a desired composition ratio of chirality. Such methods forselecting semiconducting ones and selecting chirality are described in,for example, (1) T. Tanaka et al., Applied Physics Express 2008, VOL. 1,p. 114001, (2) H. Liu et al., Nature Communications 2011,DOI:10.1038/ncomms1313, (3) A. Nish et al., Nature Nanotechnology 2007,VOL. 2, pp. 640-646, and the like. By using semiconducting carbonnanotubes whose chirality is selected, the mixing ratio of metalliccarbon nanotubes can be lower than that of carbon nanotubes that aregrown directly on a substrate. As a result, an imaging device havingexcellent characteristics can be more easily realized. In particular,with the method disclosed in the above-described literature (3),semiconducting carbon nanotubes having substantially equal diameters andhaving different chiralities can be selected. Accordingly, in order torealize an imaging device that acquires images, such as multispectralimages, in predetermined two or more wavelength ranges, it is desirableto use semiconducting carbon nanotubes whose chirality is selected bythe method disclosed in the literature (3) or the like.

In a case of fabricating an imaging device in which the photoelectricconversion layer 39 in each pixel has the same spectral sensitivitycharacteristics, a large-area application technique such as spin coatingor a doctor blading method can be used. In a case of fabricating animaging device in which each pixel has different spectral sensitivitycharacteristics, the following methods can be employed. First, one ortwo or more dispersion liquids are prepared in which semiconductingcarbon nanotubes having different chiralities are dispersed. Then, eachdispersion liquid is selectively discharged by an inkjet method. Thus,the photoelectric conversion layer 39 having different spectralsensitivity characteristics for each pixel can be formed without a mask.The photoelectric conversion layer 39 having different spectral responsecharacteristics for each pixel may alternatively be formed by employingspin coating, a doctor blading method, or the like together with a mask.

For example, semiconducting carbon nanotubes having a high ratio of achirality (8,0) can be used for a first pixel, and semiconducting carbonnanotubes having a high ratio of a chirality (7,2) can be used for asecond pixel. In this case, the first pixel has a higher sensitivity tolight at a wavelength of 780 nm than the second pixel, and the secondpixel has a higher sensitivity to light at a wavelength of 820 nm thanthe first pixel. Thus, by comparing images formed by each of the twokinds of pixels with each other, the oxidation-reduction degree ofhemoglobin can be visualized as an image. Alternatively, semiconductingcarbon nanotubes having a high ratio of a chirality (12,4), (10,6),(13,0), or the like can be used for the first pixel, and semiconductingcarbon nanotubes having a chirality (8,7) or the like can be used forthe second pixel. In this case, it is possible to acquire images at andnear the wavelength band that is deficit in sunlight.

The charge separation material 106 can be, for example, a commerciallyavailable material. Alternatively, the charge separation material 106may be provided on the interlayer insulating layer 63C by evaporation.As described above, in order to increase the distribution density of thecharge separation material 106 on the pixel electrode 32 side and tosuppress crosstalk among pixels, it is desirable to first provide thecharge separation material 106 on the interlayer insulating layer 63Cand to provide the semiconducting carbon nanotube 105 thereon.

Then, on the photoelectric conversion layer 39, the electron blockinglayer 101 is formed by the same method as the hole blocking layer 103.

Then, the counter electrode 38 is formed. For example, ITO is formed bya sputtering method. Then, the protective layer 72 and microlenses 74are formed, and thereby the imaging device is completed. In the abovemanner, with the method for fabricating the imaging device according tothe present disclosure, the photoelectric conversion unit 30 can befabricated without using crystallinity on the surface of a support bodythat supports the photoelectric conversion unit 30. In addition, theformation of the photoelectric conversion unit 30 does not include astep that needs high-temperature processing. Thus, the photoelectricconversion unit 30 can be stacked on a semiconductor substrate on whicha signal detection circuit is provided, and a stacked imaging device canbe realized.

Second Embodiment

A second embodiment is different from the first embodiment in that thecharge separation material separates holes or electrons that are used assignal charges among hole-electron pairs generated in carbon nanotubes.

Carbon nanotubes typically have a length of about several tens ofnanometers to several millimeters. In contrast, the thickness of thephotoelectric conversion layer in the photoelectric conversion elementis typically several nanometers to several hundreds of nanometers. Thus,if carbon nanotubes are used as a photoelectric conversion material forthe photoelectric conversion element, carbon nanotubes disposed inparallel (in the direction perpendicular to the thickness direction) tothe photoelectric conversion layer are longer than carbon nanotubesdisposed in the thickness direction, in the photoelectric conversionlayer.

In order to cause light to be incident on the photoelectric conversionlayer and to efficiently capture holes or electrons among generatedhole-electron pairs as signal charges, voltage is desirably applied tothe photoelectric conversion layer. FIG. 8 schematically illustrates acarbon nanotube 500 located in the photoelectric conversion layer. Apositive charge h is present at the position 501 of the carbon nanotube500, and an electric field is applied from the outside in the directionof the arrow 502. If the charge h is present in a free space, the chargeh moves to a position 511 in parallel to the arrow 502.

However, if the charge h is present in the carbon nanotube 500, thecharge h has to first exit from the carbon nanotube 500 to the outside.Thus, the charge h has to overcome an energy barrier, based on astochastic phenomenon.

On the other hand, the electric field indicated by the arrow 502 haselectric field components in a longitudinal direction 503 of the carbonnanotube 500. It is known that the charge movement along thelongitudinal direction of the carbon nanotube 500 receives littleresistance, and it has been reported that the mobility in this directionis 10000 cm²/(V·s) or higher. Accordingly, unless there are no electricfield components in parallel to the longitudinal direction of the carbonnanotube 500, the charge h moves in the carbon nanotube 500 with ease toa position 521.

The imaging device includes a plurality of pixels and forms images bygenerating signals in proportion to the amount of light that is incidenton each pixel. If carbon nanotubes are used for the photoelectricconversion layer, from the above-described reasons, charges generatedthrough photoelectric conversion move in the carbon nanotubes. Thus, ifthe length of a carbon nanotube is greater than the size of a pixel 505,when a charge generated through photoelectric conversion moves in thecarbon nanotube, a pixel that is different from the pixel on which lighthas been incident might detect the incident light. Even if the length ofa carbon nanotube is smaller than the size of the pixel 505, the sameapplies when the carbon nanotube is located to extend over two pixels.As a result, crosstalk of the detected signal occurs among pixels, and aresultant image might blur, not correctly reflecting the distribution ofan original illuminated light amount.

This issue becomes more apparent if the pitch between pixels is narroweras a result of a small size (angle of view) of the imaging device or alarge number of pixels in accordance with high resolution of the imagingdevice. The imaging device according to this embodiment solves thisissue and is particularly desirably used for an imaging device in whichthe pitch between pixels is narrow.

The general configuration of the imaging device according to thisembodiment is the same as that of the imaging device according to thefirst embodiment illustrated in FIG. 1 , for example. In addition, thedevice configuration of the imaging device according to this embodimentis the same as that of the imaging device according to the firstembodiment illustrated in FIG. 2 , for example.

Next, the photoelectric conversion layer 39 in the photoelectricconversion unit 30 of the imaging device according to this embodimentwill be described in detail. FIG. 9 schematically illustrates across-section of the photoelectric conversion unit 30 that extends overa plurality of unit pixel cells 20. In each unit pixel cell 20, thephotoelectric conversion unit 30 includes the counter electrode 38, thepixel electrode 32, and the photoelectric conversion layer 39. Thephotoelectric conversion layer 39 is sandwiched between the counterelectrode 38 and the pixel electrode 32.

In order to suppress the generation of leak current, the photoelectricconversion layer 39 is not separate for each pixel and is formed toextend over the plurality of unit pixel cells 20. The photoelectricconversion layer 39 contains the semiconducting carbon nanotubes 105 andthe charge separation material 106. At least some of the carbonnanotubes 105 are located to extend over the plurality of unit pixelcells 20.

The structure, physical properties, and the like of the carbon nanotubes105 are as described in the first embodiment. In addition, any of thematerials described in the first embodiment can be used as the chargeseparation material 106. That is, the charge separation material 106 maybe a substance having a larger electron affinity than the semiconductingcarbon nanotubes 105 or a substance having a smaller ionization energythan the semiconducting carbon nanotubes 105.

However, unlike in the first embodiment in which charges remaining inthe carbon nanotubes are caused to move to the pixel electrode to beused as signal charges, in the second embodiment, charges separated bythe charge separation material from the carbon nanotubes are caused tomove to the pixel electrode. For example, if electrons, which arenegative charges, are used as signal charges, the voltage supply circuit10 supplies a bias voltage to the counter electrode 38 in such a mannerthat the potential of the pixel electrode 32 is higher than thepotential of the counter electrode 38. In this case, a substance havinga larger electron affinity than the semiconducting carbon nanotubes 105can be used as the charge separation material 106. In this case, amonghole-electron pairs generated in the semiconducting carbon nanotubes105, electrons move to the conduction band 110 of the charge separationmaterial 106 at a lower energy level. Thus, holes remain in the valenceband of the semiconducting carbon nanotubes.

Since a bias voltage that is set in such a manner that the potential ofthe pixel electrode 32 is higher than the potential of the counterelectrode 38 is supplied, electrons move from the charge separationmaterial 106 to the pixel electrode 32. If holes, which are positivecharges, are used as signal charges, the voltage supplying circuitsupplies a bias voltage to the counter electrode 38 in such a mannerthat the potential of the pixel electrode 32 is lower than the potentialof the counter electrode 38. In this case, a substance having a smallerionization energy than the semiconducting carbon nanotubes 105 can beused as the charge separation material 106. In this case, the chargeseparation material 106 is more stabilized by becoming a positive ion.Thus, among hole-electron pairs generated in the semiconducting carbonnanotubes 105, holes move to the valence band of the charge separationmaterial 106. Accordingly, electrons remain in the conduction band 110of the semiconducting carbon nanotubes 105. These holes move to thepixel electrode. Since a bias voltage that is set in such a manner thatthe potential of the pixel electrode 32 is lower than the potential ofthe counter electrode 38 is supplied, the holes move from the chargeseparation material 106 to the pixel electrode 32.

In order to suppress issues caused by one-dimensional charge movement inthe semiconducting carbon nanotubes 105, it is desirable that the chargemovement in the charge separation material 106 not be one-dimensional.For example, in fullerene, fullerene derivatives, and most of lowmolecules, charges are restricted by molecular orbits. Thus, by applyingan electric field from the outside, charges are biased in molecules, butthe charge movement that generates current in a macroscopic view doesnot occur. That is, there is no degree of freedom in charge movement inthe charge separation material 106 in a macroscopic view. In this case,the charge movement in the charge separation material 106 iszero-dimensional. In addition, a large number of conductive polymershave one-dimensional chain molecular shapes by a covalent bond and havea structure in which these chains are complicatedly tangled. In suchconductive polymers, charges move by hopping conduction along themolecular chains and between molecular chains at positions that arespatially adjacent to each other due to tangles. The difference betweenthe efficiencies of the two kinds of charge movement is much smallerthan that in carbon nanotubes. Accordingly, charge movesthree-dimensionally in conductive polymers.

In the photoelectric conversion layer 39, it is desirable that both thesemiconducting carbon nanotubes 105 and the charge separation material106 be dispersed. As long as both are dispersed, the distribution of thesemiconducting carbon nanotubes 105 and the charge separation material106 do not have to be uniform. For example, the distribution may bebiased in the thickness direction of the photoelectric conversion layer39. Specifically, unlike in the first embodiment, the density of thesemiconducting carbon nanotubes 105 in the thickness direction of thephotoelectric conversion layer 39 may be high on the counter electrode38 side and may be low on the pixel electrode 32 side. In addition, thedensity of the charge separation material 106 in the thickness directionof the photoelectric conversion layer 39 may be low on the counterelectrode 38 side and may be high on the pixel electrode 32 side. Suchdistribution is more desirable than the inverse density distribution inorder for the charge separation material 106 to separate charges to becaptured by the pixel electrode 32 from the semiconducting carbonnanotubes. The thickness of the photoelectric conversion layer 39 is,for example, greater than or equal to several tens of nanometers andless than or equal to several hundreds of nanometers. In addition, thephotoelectric conversion layer 39 may further contain an n-typesemiconductor or a p-type semiconductor.

As in the first embodiment, the photoelectric conversion unit 30 mayfurther include the hole blocking layer 103 and the electron blockinglayer 101. The hole blocking layer 103 and the electron blocking layer101 can be formed of any of the materials described in the firstembodiment.

The detection of incident light in the photoelectric conversion layer 39will be described with reference to FIG. 10 . FIG. 10 schematicallyillustrates the positional relationship between the arrangement of asemiconducting carbon nanotube 105 and the charge separation material106 located in the photoelectric conversion layer 39 and pixelelectrodes 32 of unit pixel cells. The semiconducting carbon nanotube105 is located to extend over two unit pixel cells 20. In the followingexample, electrons are used as signal charges as in the firstembodiment. However, unlike in the first embodiment, the chargeseparation material 106 is a substance having a larger electron affinitythan the semiconducting carbon nanotube 105. In addition, in FIG. 10 , abias voltage is supplied to the counter electrode 38 in such a mannerthat the potential of the pixel electrode 32 is higher than thepotential of the counter electrode 38. Thus, an electric field isgenerated in the direction indicated by the arrow 502.

As in the first embodiment, upon absorption of a photon (notillustrated) by the semiconducting carbon nanotube 105, a positivecharge (hole) h and a negative charge (electron) e are generated at theposition 501 that is substantially the same as the position of theabsorption. The generated positive charge h and negative charge e haveattractive forces with respect to each other, creating a state called anexciton. Excitons are electrically neutral in a macroscopic view andthus only move gradually so as to be dispersed, the positions of theexcitons not being influenced by the applied electric field.

Upon an exciton arriving at a position close to the charge separationmaterial 106, the charges of the exciton are separated. Specifically,the charge separation material 106 separates the negative charge e, sothat one of the charges moves to one of the molecules of the chargeseparation material 106, and the other charge remains in thesemiconducting carbon nanotube 105. In the example illustrated in FIG. 7, since the charge separation material 106 has a larger electronaffinity than the semiconducting carbon nanotube 105, the chargeseparation material 106 separates the negative charge e. As a result,the positive charge remains in the semiconducting carbon nanotube 105.

The charges are separated at the position where the semiconductingcarbon nanotube 105 is close to the charge separation material 106.Accordingly, if one or more molecules of the charge separation material106 are present within a unit pixel cell 20 in the periphery of one ofthe molecules of the semiconducting carbon nanotube 105, the charges areseparated in the pixel where the exciton has been generated. In order tocause the exciton to be generated and the charges to be separated withinthe same unit pixel cell, the ratio of the charge separation material106 contained in the photoelectric conversion layer 39 is desirablyincreased.

The positive charge h remaining in the semiconducting carbon nanotube105 and the negative charge e that has moved to the charge separationmaterial 106 are no longer neutral even in a macroscopic view.Accordingly, both the positive charge h and the negative charge e areinfluenced by the electric field indicated by the arrow 502 due to thevoltage applied between the counter electrode 38 and the pixel electrode32 and start to move.

The positive charge h remaining in the semiconducting carbon nanotube105 moves by being strongly influenced by electric field components notonly in the direction of the arrow 502 but also in the direction 503along which the semiconducting carbon nanotube 105 extends, due to theanisotropy of electric conduction of a carbon nanotube. As a result, thepositive charge h moves in the direction along which the semiconductingcarbon nanotube 105 extends to arrive at the position 521. The positivecharge h is captured by the counter electrode 38, which is notillustrated. Since the counter electrode 38 is formed to extend over aplurality of pixels, the positive charge h may be captured at anyposition.

On the other hand, the negative charge e that has moved to the one ofthe molecules of the charge separation material 106 moves to an adjacentmolecule of the charge separation material 106 by hopping conduction.The molecule to which the positive charge h hops is determined by thedistance between molecules and the direction and strength of theelectric field. If the molecules have substantially equal distancestherebetween, a molecule in a direction that is closer to the directionparallel to the electric field is selected as the hopping destination.If the strength of the electric field is strong, even though distancesbetween molecules are different, the possibility that a molecule in adirection that is closer to the direction parallel to the electric fieldis selected as the hopping destination is higher.

Accordingly, if the distribution of distances between molecules of thecharge separation material 106 is not largely biased and the chargeseparation material 106 is distributed three-dimensionally in thephotoelectric conversion layer 39, the negative charge e moves in thedirection of the electric field in a macroscopic view. A case where thedistribution of distances between molecules is largely biased is, forexample, a case where the molecules of the charge separation material106 are arranged only on a certain plane. Accordingly, the negativecharge e is captured by the pixel electrode 32 in a unit pixel cell 20where the exciton has been generated. That is, the pixel on which lighthas been incident is the same as the pixel that detects the incidentlight.

Even if the charge movement in the charge separation material 106 isthree-dimensional, charges move in the direction of the electric fieldin a macroscopic view. In addition, by increasing the difference inpotential to be applied between the counter electrode 38 and the pixelelectrode 32 and increasing the strength of the electric field, anegative charge can be more reliably detected in a pixel where anexciton has been generated.

Although an example where the negative charge e is captured by the pixelelectrode 32 has been described with reference to FIG. 10 , the casewhere the positive charge h is captured by the pixel electrode 32 can beexplained in the same manner. In addition, although FIG. 10 illustratesthe single semiconducting carbon nanotube 105, two or moresemiconducting carbon nanotubes 105 might be close to each other in thephotoelectric conversion layer 39. In this case, in addition to theabove-described (i) charge separation between the semiconducting carbonnanotube 105 and the charge separation material 106 and (ii) chargemovement among molecules of the charge separation material 106, (iii)charge separation between the two or more semiconducting carbonnanotubes 105 and (iv) charge movement from the semiconducting carbonnanotubes 105 to the charge separation material 106 may occur. However,the semiconducting carbon nanotubes 105 having the same chirality haveequal electron affinity and ionization energy. In addition, from theview point of the electron affinity or ionization energy, a positivecharge or negative charge to be detected as a signal advantageouslymoves from the semiconducting carbon nanotubes 105 to the chargeseparation material 106 in terms of energy. Accordingly, the positivecharge or negative charge is difficult to move from the chargeseparation material 106 to the semiconducting carbon nanotubes 105.Thus, charge movement and the like in (iii) and (iv) are disadvantageousover (i) and (ii) in terms of energy and are difficult to occur.

In addition, as is clear from the above description, if the chargeseparation material 106, even in a small content, is contained in thephotoelectric conversion layer 39, a charge to be detected as a signalin the semiconducting carbon nanotubes 105 can be separated from thesemiconducting carbon nanotubes 105 and can be moved toward the pixelelectrode 32 in accordance with the electric field applied between thecounter electrode 38 and the pixel electrode 32. Accordingly, theimaging device according to the present disclosure can produce theabove-described effects in accordance with the content of the chargeseparation material 106 in the photoelectric conversion layer 39.

In the above manner, according to this embodiment, in addition to theeffects described in the first embodiment, an imaging device that cansuppress mixing of detected charges among pixels and that can acquirehigh-definition images or an imaging device having a minute pixel sizecan be realized.

The imaging device according to this embodiment can be fabricatedthrough a typical semiconductor fabrication process as in the imagingdevice according to the first embodiment. In particular, if a siliconsubstrate is used as the semiconductor substrate 62, the imaging devicecan be fabricated through a variety of silicon semiconductor processes.

Note that the above embodiments have described examples in which each ofthe amplifier transistor 42, the address transistor 44, and the resettransistor 46 is an n-channel MOS transistor. However, the transistorsin the embodiments of the present disclosure are not limited ton-channel MOS transistors. The amplifier transistor 42, the addresstransistor 44, and the reset transistor 46 may be p-channel MOStransistors. In addition, all of the amplifier transistor 42, theaddress transistor 44, and the reset transistor 46 do not have to ben-channel MOS transistors or p-channel MOS transistors. As a transistor,an FET or a bipolar transistor can also be used.

In addition, each of the above embodiments has described an imagingdevice having a mode in which the pixel electrode captures a charge tobe detected. However, the imaging device may include an impuritydiffused region provided in the semiconductor substrate instead of thepixel electrode, and the impurity diffused region may capture a hole oran electron among hole-electron pairs generated in the photoelectricconversion layer as a charge to be detected.

The imaging device according to the present disclosure can desirably beused as a variety of imaging devices that can acquire images in thevisible region and in the near-infrared region. In particular, theimaging device can be desirably used as an imaging device with ahigh-speed operation and a high frame rate, a high-definition imagingdevice with a large number of pixels, and/or an imaging device with aminute pixel size.

What is claimed is:
 1. An imaging device comprising: pixels, each of thepixels including: a photoelectric conversion layer that containssemiconductor carbon nanotubes absorbing light and generating electriccharges; and a charge capturing unit that captures the electric chargesgenerated by the semiconductor carbon nanotubes; and an electroniccircuit that converts the electric charges captured by the chargecapturing unit into a signal representing an image, wherein the pixelsinclude a first pixel and a second pixel, and a chirality of thesemiconductor carbon nanotubes included in the first pixel is differentfrom a chirality of the semiconductor carbon nanotubes included in thesecond pixel, such that the first pixel and the second pixel have asensitivity to light at different wavelengths.
 2. The imaging deviceaccording to claim 1, wherein the photoelectric conversion layer furthercontains one of a first substance and a second substance, and thesemiconductor carbon nanotubes have a chirality such that the firstsubstance has an electron affinity larger than that of thesemiconducting carbon nanotubes, or the second substance has anionization energy smaller than that of the semiconductor carbonnanotubes.
 3. The imaging device according to claim 2, wherein thephotoelectric conversion layer contains the first substance, and thecharge capturing unit captures positive charges.
 4. The imaging deviceaccording to claim 2, wherein the photoelectric conversion layercontains the second substance, and the charge capturing unit capturesnegative charges.
 5. The imaging device according to claim 1, furthercomprising: a semiconductor substrate that supports the photoelectricconversion layer, wherein the electronic circuit includes a chargedetecting transistor that is provided on the semiconductor substrate andis electrically connected to the charge capturing unit.
 6. The imagingdevice according to claim 5, wherein each of the pixels further includesan impurity diffusion region in the semiconductor substrate, theimpurity diffusion region electrically connected to the photoelectricconversion layer.
 7. The imaging device according to claim 5, whereinthe charge capturing unit is an electrode located above thesemiconductor substrate.
 8. The imaging device according to claim 1,wherein the photoelectric conversion layer is continuous among thepixels.
 9. The imaging device according to claim 8, wherein at least oneof the semiconductor carbon nanotubes is located to extend over twopixels of the pixels.
 10. The imaging device according to claim 1,wherein the semiconductor carbon nanotubes have the chirality such thatthe semiconductor carbon nanotubes have an absorption peak in anear-infrared region, and the image is a near-infrared image.
 11. Theimaging device according to claim 10, wherein the absorption peak existsin a wavelength band that is deficient in sunlight.
 12. The imagingdevice according to claim 1, wherein the photoelectric conversion layerfurther contains a charge separation material having a fullereneskeleton.
 13. The imaging devise imaging device according to claim 1,wherein the photoelectric conversion layer is disposed between anelectrode and the charge capturing unit, and a density of thesemiconductor carbon nanotubes is higher on the electrode side than onthe charge capturing unit side.
 14. The imaging devise imaging deviceaccording to claim 1, wherein the photoelectric conversion layer isdisposed between an electrode and the charge capturing unit, and adensity of the semiconducting carbon nanotubes is higher on the chargecapturing unit side than on the electrode side.
 15. An imaging devicecomprising: pixels, each of the pixels including: a photoelectricconversion layer that contains semiconductor carbon nanotubes absorbinglight and generating electric charges; and a charge capturing unit thatcaptures the electric charges generated by the semiconductor carbonnanotubes; and an electronic circuit that converts the electric chargescaptured by the charge capturing unit into a signal representing aTime-of-Flight image.
 16. An imaging device comprising: pixels, each ofthe pixels including: a photoelectric conversion layer that containssemiconductor carbon nanotubes absorbing light and generating electriccharges, and at least one charge separation material; and a chargecapturing unit that captures the electric charges generated by thesemiconductor carbon nanotubes; and an electronic circuit that convertsthe electric charges captured by the charge capturing unit into a signalrepresenting an image, wherein the semiconductor carbon nanotubesinclude two or more kinds of semiconductor carbon nanotubes havingdifferent chirality, and the at least one charge separation materialincludes two or more kinds of charge separation materials.